Plasmid vectors and use of plasmid vectors in methods for producing methane and isoprene using Archaea are disclosed. Particularly, plasmid vectors that express isoprene synthase (ispS) are prepared and inserted into methanogens, such as Methanosarcina acetivorans, to allow for co-production of methane and isoprene. In one embodiment, the methods of the present disclosure can be used for wastewater management.